FIG. 1 shows parts of a wireless communications system, in which a plurality of source user equipments UE1, UE2, UE3 are in communication with a plurality of destination user equipments UE4, UE5, UE6 via a base station BS. Source UEs UE1, UE2, UE3 transmit data packets in an uplink to the base station BS. Base station BS receives the data packets from the source UEs and buffers these data packets for onward transmission in a downlink to the destination UEs. In this example, each wireless channel is allocated a separate channelisation code in order to distinguish that channel from the other channels (CDMA).
In the system of FIG. 1 it is necessary to provide some mechanism for determining when and how each of the source UEs transmits its data in the uplink to the base station. In the simplest case, each source UE transmits whenever it has data to send. This technique may work well when there are low offered loads on the uplink. However, if too many UEs try to transmit data at the same time, the interference levels may become unacceptable, resulting in poor quality of service.
Various techniques are known for scheduling of uplink transmissions. The aim of these scheduling techniques is to manage the way in which the UEs transmit to the base station in order to meet certain criteria, such as interfere levels, fairness or throughput.
In one scheduling technique, known as time scheduling, a single UE is given the full uplink resources for a given period of time. Some mechanism is implemented for determining which UE has the channel resources at any one time. For example, each UE may take it in turns to transmit its data, or else the UE which is given the channel resources may be selected by taking into account the channel quality.
In time scheduling, only one UE transmits at any one time, and so each UE can transmit within its time window at a high data rate, without causing interference to other UEs. However, time scheduling is inefficient in terms of throughput in the uplink due to the delays involved in informing each UE when it can transmit. Other disadvantages of time scheduling include loss of interference diversity (which increases the interference caused to adjacent cells), less accurate power control, due to less frequent power control signals, and wasted uplink capacity when a UE has little to transmit.
In another scheduling technique, known as rate scheduling, each UE decides on the rate at which it transmits data based on information which is signalled to it by the network. For example, in 3G communications systems (W-CDMA), each UE may decide its rate based on information from the Transport Format Combination Set (TFCS) signalled to it by the radio network controller (RNC). This rate is usually less than the UE's maximum rate, and is set to ensure that interference remains within acceptable levels.